In a known toggle press a hydraulic member is connected between the force transmitting and force receiving components, such as the toggle joint, carriage and platen of the press. A hydraulic chamber for the hydraulic member acts as a press cushion which is continuously connected with a fluid reservoir or accumulator which is maintained under gas pressure.
A mechanical press of this basic type is described in German Open Patent Application DE-OS No. 27 32 278 in which a hydraulic intermediate member is connected to the platen, plunger and connecting rod whose pressure during overload at the lower dead point can be relieved by opening a valve.
The valve in the hydraulic intermediate member controlling the pressure dependence is designed as an electrically controllable valve which is connected electrically by a control mechanism with at least one electrical sensor which is positioned so that it responds to a predetermined stress in one of the structural components under pressure.
Of course this structure for the press results in the force receiving and transmitting components having the same rigidity under both small and large pressing forces because the same pressure is maintained constantly in operation of the hydraulic system. However since the electrically controllable valve opens when a predetermined load has been exceeded and the working hydraulic fluid flows away from the hydraulic member not under pressure, the operating cycle of the press is interrupted and thus a jamming or an overload of the press at its lower dead point is counteracted.
In many cases, especially in operation of a calibrating or sizing press for a prefabricated material made in a sintering process, it is desirable or even required that it be subjected to a precisely preset pressing force, independent of differing height measurements of the article to be sized resulting from the prefabrication within certain limits.
The processing of the material in this calibrating process must be such that the so-called "spring back effect" is prevented or at least is kept within certain definite limits. The "spring back effect" can lead in an undesirable way to uncertainties and variations in the dimensions of the calibrated surfaces of the product from those dimensions preset in the calibrating device.
Essentially in this calibrating process the operating cycle of the press should run on while the pressing force is maintained at its full value when the initial measurements of the piece to be calibrated exceed those aimed at in the prefabrication process.